1. Field of the Invention
The present invention relates to a metallization process for manufacturing semiconductor devices. More particularly, the present invention relates to the metallization of semiconductor substrates having apertures to form void-free interconnections between conducting layers, and planar metal surfaces.
2. Description of the Related Art
Sub-half micron multilevel metallization is one of the key technologies for the next generation of very large scale integration (xe2x80x9cVLSIxe2x80x9d). The multilevel interconnections that lie at the heart of this technology require planarization of high aspect ratio apertures, including contacts, vias, lines or other features. Reliable formation of these interconnects is very important to the success of VLSI and to the continued effort to increase circuit density and quality on individual substrates and die.
Aluminum (Al) or copper (Cu) layers formed by chemical vapor deposition (xe2x80x9cCVDxe2x80x9d), like other CVD processes, provide good conformal layers, i.e., a uniform thickness layer on the sides and base of the feature, for very small geometries, including sub-half micron ( less than 0.5 xcexcm) apertures, at low temperatures. Therefore, CVD processes (CVD Al or CVD Cu) are common methods used to fill apertures. However, transmission electron microscopy data (xe2x80x9cTEMxe2x80x9d) has revealed that voids exist in many of the CVD formed Al apertures even though electric tests of these same apertures do not evidence the existence of this void. If the layer is subsequently processed, the void can result in a defective circuit. It should be recognized that this kind of void is very difficult to detect by regular cross sectional standard electron microscopy (xe2x80x9cSEMxe2x80x9d) techniques, because some deformation occurs in soft aluminum during mechanical polishing. In addition, electric conductivity tests do not detect any structural abnormalities. However, despite generally positive electric conductivity tests, conduction through the contact having the void may, over time, compromise the integrity of the integrated circuit devices.
A TEM study of various CVD Al layers formed on substrates indicates that the formation of voids occurs through a key hole process wherein the top portion of the via becomes sealed before the via has been entirely filled. Although a thin conformal layer of CVD Al can typically be deposited in high aspect ratio contacts and vias at low temperatures, continued CVD deposition to complete filling of the contacts or vias typically results in the formation of voids therein. Extensive efforts have been focused on elimination of voids in metal layers by modifying CVD processing conditions.
An alternative technique for metallization of high aspect ratio apertures, is hot planarization of aluminum through physical vapor deposition (xe2x80x9cPVDxe2x80x9d). The first step in this process requires deposition of a thin layer of a refractory metal such as titanium (Ti) on a patterned wafer to form a barrier/wetting layer which facilitates flow of the Al during the PVD process. Following deposition of the barrier/wetting layer, the next step requires deposition of either (1) a hot PVD Al layer or (2) a cold PVD Al layer followed by a hot PVD Al layer onto the wetting layer. However, hot PVD Al processes are very sensitive to the quality of the wetting layer, wafer condition, and other processing parameters. Small variations in processing conditions and/or poor coverage of the wetting layer can result in incomplete filling of the contacts or vias, thus creating voids. In order to reliably fill the vias and contacts, hot PVD Al processes must be performed at temperatures above about 450xc2x0 C. Even at higher temperatures, PVD processes may result in a bridging effect whereby the mouth of the contact or via is closed because the deposition layer formed on the top surface of the substrate and the upper walls of the contact or via join before the floor of the contact or via has been completely filled.
Once a PVD Al layer has been deposited onto the substrate, reflow of the Al may occur by directing ion bombardment towards the substrate itself. Bombarding the substrate with ions causes the metal layer formed on the substrate to reflow. This process typically heats the metal layer as a result of the energy created by the plasma and resulting collisions of ions onto the metal layer. The high temperatures generated in the metal layers formed on the substrate compromises the integrity of devices having sub-half micron geometries. Therefore, heating of the metal layers is disfavored in these applications.
U.S. Pat. No. 5,147,819 (xe2x80x9cthe ""819 patentxe2x80x9d) discloses a process for filling vias that involves applying a CVD Al layer with a thickness of from 5 percent to 35 percent of the defined contact or via diameter to improve step coverage, then applying a sufficiently thick PVD Al layer to achieve a predetermined overall layer thickness. A high energy laser beam is then used to melt the intermixed CVD Al and PVD Al and thereby achieve improved step coverage and planarization. However, this process requires heating the wafer surface to a temperature no less than 660xc2x0 C. Such a high temperature is not acceptable for most sub-half micron technology. Furthermore, the use of laser beams scanned over a wafer may affect the reflectivity and uniformity of the metal layer.
Other attempts at filling high aspect ratio sub-half micron contacts and vias using known reflow or planarization processes at lower temperatures have resulted in dewetting of the CVD Al from the silicon dioxide (SiO2) substrate and the formation of discontinuous islands on the side walls of the vias. Furthermore, in order for the CVD Al to resist dewetting at lower temperatures, the thickness of the CVD Al has to be several thousand Angstroms (A). Since ten thousand Angstroms equal one micron, a CVD Al layer of several thousand Angstroms on the walls of a sub-half micron via will completely seal the via and form voids therein.
U.S. Pat. No. 5,665,659, describes a method for forming a metal layer on a semiconductor substrate including depositing a barrier/wetting layer, heat treating the substrate for a predetermined time at an intermediate temperature between 200xc2x0 C. and 400xc2x0 C., and then depositing a PVD metal layer on the semiconductor substrate at a temperature below 200xc2x0 C. and a pressure below about 2 milliTorr. The deposited metal layer is then thermally treated at a temperature between 0.6 Tm-1.0 Tm (where Tm is the melting point of the metal layer) to reflow the metal layer. The barrier/wetting layer is heat treated and the metal layer is carefully cooled to reduce formation of grooves on the metal layer surface.
There remains a need for a metallization process for filling apertures, particularly high aspect ratio sub-half micron contacts and vias, with metal such as aluminum. More particularly, it would be desirable to have a PVD process for filling such contacts and vias.
The present invention provides a metallization process for filling apertures on a substrate. First, a thin refractory layer is deposited on a substrate followed by depositing a PVD metal layer at a pressure less than about 1 milliTorr to form a conformal layer. The conformal PVD metal layer does not fill the apertures. Then a PVD metal is deposited on the substrate and heated to reflow the metal and fill the apertures.
In one aspect of the invention, a barrier layer is deposited onto a substrate having high aspect ratio contacts or vias formed thereon. A titanium or titanium/titanium nitride barrier layer is preferred for deposition of aluminum. A conformal aluminum layer is then deposited onto the barrier layer by physical vapor deposition at a pressure less than about 1 milliTorr, preferably less than about 0.35 milliTorr. The conformal aluminum layer is preferably deposited in a sputtering chamber having a target positioned at least about 100 mm from a substrate. Next, aluminum is deposited by physical vapor deposition onto the conformal aluminum layer and the via is filled by reflow or annealing of the deposited aluminum.
In another aspect of the invention, the metallization process is carried out in an integrated processing system that includes PVD chambers for depositing a metal such as aluminum without the formation of an oxide layer over the aluminum layers. The processing system may also contain reflow chambers, preclean chambers, and barrier layer chambers associated with deposition of the metal layers.